Polycrystalline Grits and Associated Methods

ABSTRACT

A method for forming polycrystalline grits can include forming an abrasive dough, including a plurality of abrasive particles, into a sheet. The sheet can be divided into a plurality of abrasive precursors. By placing the sheet on a stretchable surface, separations among the plurality of abrasive precursors can be revealed by stretching the stretchable surface. The stretchable surface can include a particulate separating agent, and additional particulate separating agent can be distributed in the separations. The abrasive precursors can be sintered to form polycrystalline grits.

PRIORITY DATA

This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/986,532, filed on Nov. 8, 2007, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to methods for use in producing abrasive grits, and the associated grits, particularly polycrystalline grits. Accordingly, the present invention involves the fields of chemical engineering, chemistry, metallurgy, and materials science.

BACKGROUND OF THE INVENTION

Abrasive particles can be used in a variety of applications such as cutting, sawing, grinding, lapping, drilling, and polishing of materials. Selection of abrasive particles depends on the anticipated use for the particles. As such, abrasive particles come in a variety of materials. For applications requiring extraordinarily hard abrasive particles, superabrasive particles, such as diamond and cubic boron nitride (cBN), can be used. In some cases, they may be single crystal grits, or polycrystalline grits.

Creation of grits can be very time and resource intensive. It can rely on expensive materials, and can require extraordinary processing conditions, such as high pressure high temperature (HPHT) conditions. Furthermore, the grits created are generally non-uniform and require sorting for size and quality.

SUMMARY OF THE INVENTION

The present invention provides systems and methods for forming polycrystalline grits. The polycrystalline grits can be formed by first forming an abrasive dough including a plurality of abrasive particles into a sheet. At least a portion of the sheet can be placed upon a stretchable surface that includes a particulate separating agent. The sheet can also be divided into a plurality of abrasive precursors. Once the sheet is divided, the stretchable surface can be stretched to reveal separations among the plurality of abrasive precursors. Particulate separating agent can be distributed in the separations. The abrasive precursors can then be sintered to form polycrystalline grits.

Such method can be used to produce polycrystalline grits including materials such as, but not limited to, diamond, nanodiamond, cubic boron nitride, silicon carbide, quartz, corundum, silicon nitride, boron nitride, tungsten carbide, titanium carbide, zirconium carbide, zinc oxide, zirconia, alumina, aluminum nitride, titanium nitride, zirconium nitride, and mixtures or compositions thereof.

Similarly, a method of forming polycrystalline grits can include forming a plurality of abrasive precursors including a binder and a plurality of abrasive particles. The abrasive precursors can be coated with a particulate separating agent, and the abrasive precursors can be sintered to form polycrystalline grits.

In another embodiment, an abrasive precursor assembly can include a plurality of abrasive precursors and a particulate separating agent. The abrasive precursors can include sintering aid, abrasive particles and binder. Also, the abrasive precursors can be arranged on a stretchable surface stretched sufficient to reveal separations between the abrasive precursors. Particulate separating agent can be placed in the separations and between the abrasive precursors and stretchable surface.

There has thus been outlined, rather broadly, various features of the invention so that the detailed description thereof that follows may be better understood, and so that the present contribution to the art may be better appreciated. Other features of the present invention will become clearer from the following detailed description of the invention, taken with the accompanying claims, or may be learned by the practice of the invention.

DETAILED DESCRIPTION

Before the present invention is disclosed and described, it is to be understood that this invention is not limited to the particular structures, process steps, or materials disclosed herein, but is extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

It must be noted that, as used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an abrasive precursor” includes one or more of such precursors, reference to “a separating agent” includes reference to one or more of such materials, and reference to “a sintering process” includes reference to one or more of such processes.

Definitions

In describing and claiming the present invention, the following terminology will be used in accordance with the definitions set forth below.

As used herein, “polycrystalline grits” refers to small particulates having multiple crystalline structures. Typically, the polycrystalline grits are a mass of sintered single crystals (or smaller polycrystalline crystals), often with a small amount of sintering aid. Further, the term “grits” indicates a particle size within a range that is well known to those of ordinary skill in the art. However, in some aspects, such particles are most often less than about 2 mm. This is in contrast to larger polycrystalline compacts which can be up to several centimeters across and tens of millimeters thick.

As used herein, “precursor assembly” refers to an unsintered mass of abrasive particles formed in discrete abrasive precursor bodies, each substantially separated by separating agent. A precursor assembly can refer to the assembly either before or after dewaxing to remove binder and/or other organic constituents, as well as any other point between formation of the assembly and the final sintering thereof.

As used herein, “nanodiamond” refers to carbonaceous particles having crystal sizes in the nanometer range, i.e. about 1 nm to about 20 nm. Further, nanodiamond is intended to refer to particles having nanometer scale crystal structure. Nanodiamond particles can be formed using a number of known techniques. One nanodiamond formation technique involves the explosion of dynamite or other explosives to produce nanodiamond having nanocrystalline structure and has particle sizes in the range of from about 2 to about 10 nm. In contrast, typical fine diamond particles have a particle size larger than about 0.1 μm.

As used herein, “crystal” is to be distinguished from “particle.” Specifically, a crystal refers to a structure in which the repeated or orderly arrangement of atoms in a crystal lattice extends uninterrupted, although defects may be present. Many crystalline solids are composed of a collection of multiple crystals or grains. A particle can be formed of a single crystal or from multiple crystals as individual crystals grow sufficient that adjacent crystals impinge on one another to form grain boundaries between crystals. Thus, each crystal within a polycrystalline particle can have a random orientation.

As used herein, “dewaxing” refers to a heating process for removing organic constituents from a mass of abrasive particles and/or metallic material. As such, dewaxing is not limited to removal of waxes and/or paraffins, but can encompass any solvent removal, e.g., water or other liquid thinners such as those that merely evaporate upon heating.

As used herein, a plurality of components may be presented in a common list for convenience. However, these lists should be construed as though each member of the list is individually identified as a separate and unique member. Thus, no individual member of such list should be construed as a de facto equivalent of any other member of the same list solely based on their presentation in a common group without indications to the contrary.

Concentrations, amounts, particle sizes, volumes, and other numerical data may be expressed or presented herein in a range format. It is to be understood that such a range format is used merely for convenience and brevity and thus should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited.

As an illustration, a numerical range of “about 1 to about 5” should be interpreted to include not only the explicitly recited values of about 1 to about 5, but also include individual values and sub-ranges within the indicated range. Thus, included in this numerical range are individual values such as 2, 3, and 4 and sub-ranges such as from 1-3, from 2-4, and from 3-5, etc. This same principle applies to ranges reciting only one numerical value. Furthermore, such an interpretation should apply regardless of the breadth of the range or the characteristics being described.

The Invention

It has been found that polycrystalline abrasive grits can be produced by a method which can improve speed of production, as well as production efficiency. Further, such method can produce grits having predetermined shapes.

In accordance with embodiments presented herein, various details are provided which are applicable to each of the methods for producing polycrystalline abrasive grits, methods for producing related abrasive precursors, polycrystalline abrasive grits, abrasive precursors, and assemblies of grits and precursors. Thus, discussion of one specific embodiment is related to and provides support for this discussion in the context of the other related embodiments in many respects.

In one embodiment, a method for forming polycrystalline grits includes forming an abrasive dough including abrasive particles. The dough can be divided into a plurality of abrasive precursors and placed on a stretchable surface. The stretchable surface can have a particulate separating agent placed thereon, which can separate, at least partially, the stretchable surface from the abrasive dough sheet, divided or undivided. With the abrasive precursors on the stretchable surface, the stretchable surface can be stretched to reveal separations among the abrasive precursors. Particulate separating agent can then be distributed in the separations. The abrasive precursors can then be sintered into a plurality of polycrystalline grits.

An abrasive dough can be formed with a plurality of abrasive particles. The abrasive particles can be selected from any abrasive particle which can be useful for removing materials from a workpiece. Non-limiting examples of abrasive particles that may be used include diamond, nanodiamond, cubic boron nitride, silicon carbide, quartz, corundum, i.e. sapphire, silicon nitride, boron nitride, tungsten carbide, titanium carbide, zirconium carbide, zinc oxide, zirconia, alumina, aluminum nitride, titanium nitride, zirconium nitride, and mixtures or composites thereof. The selection of the abrasive particles can be determined, at least in part, by the intended use of the polycrystalline grits. In one embodiment, for example, nanodiamonds can be included for a polishing use, whereas superabrasive particles can be used. In a specific embodiment, diamond particles can be used. The abrasive particles can be homogeneously selected, or can include more than one type of abrasive particle. For example, both abrasive and superabrasive particles can be used, or, for example, both diamond and cubic boron nitride could be used in combination.

In another embodiment, the dough can include abrasive particles and superabrasive particles. This allows for decreased cost and increased impact strengths. Suitable abrasive particles can include, but are not limited to, titanium carbide, titanium nitride, aluminum nitride, silicon carbide, silicon nitride, silicon carbide, zirconium, alumina, silicon oxide, or mixtures of these particles.

In one aspect, the abrasive particles can be diamond particles. Diamond superabrasive particles can be useful in a wide variety of applications. Typically, diamond can be suitable for applications which require high removal rates for relatively hard materials such as granite, metals, or the like. In another alternative embodiment, the polycrystalline grits can include both diamond and cBN particles in various proportions. This composite polycrystalline composition can be useful, for example, for cutting steel and non-steel compositions, e.g., steel reinforced concrete.

In another aspect, the abrasive particles can be nanodiamond particles, e.g. nanodiamond particles produced from explosion synthesis. These nanodiamond particles are not produced by pulverizing or milling larger particles. Without being bound to any particular theory, it appears that nanodiamond particles formed by explosion synthesis are formed in situ via crystallization of a liquid solution of diamond. Additionally, explosion synthesis nanodiamond particles include a high content of defects. These defects are on a sub-nanoscale and tend to allow the nanodiamond particles to cleave and chip along these sub-nanoscale defects during use. Thus, as the nanodiamond particles become worn, the particles can chip and break to expose sub-nanoscale cutting surfaces. As a result, such nanodiamond particles do not slide along the surface of a workpiece, but rather cut and remove material on a sub-nanoscale.

Prior to formation of the dough, the abrasive particles can optionally be cleaned. This can help to improve sintering of particles by removing foreign material which can interfere with contact and growth of particles during sintering. For example, the abrasive particles can be mixed in an organic solvent, e.g., trimethylethane; an acid, e.g., HCl; and a fused salt, e.g., NaNO₃. The abrasive particles can then be rinsed in deionized water and dried.

Once the abrasive particles are optionally cleaned, they can be included in the dough. The dough can be formed by combining abrasive particles and a binder. A dough is malleable, yet substantially maintains its shape without external assistance. In this manner, the dough of the present application differs from most slurries in that slurries are typically fluid and become the shape of the object in which they are placed. A dough, on the other hand, is able to be shaped to a desired configuration, and can remain as a cohesive mass. While external forces and shaping devices may be used with the dough, they are not required. Dough of the present application can be processed to the desired shape through methods which would be apparent to one of skill in the art. In one aspect, dough can be extruded one or a plurality of times to produce a sheet. Also or alternatively, a roller can be used one or more times to flatten the dough into a sheet. In this embodiment, one or a plurality of sheets can be combined to form a single sheet that can be used to form the abrasive particles.

The dough can, in some aspects, include a sintering aid. The sintering aid can be any material capable of enhancing sintering of the abrasive particles. Non-limiting examples of suitable sintering aids for diamond materials include Fe, Co, Ni, Mn, Cr, and alloys thereof Alloys of iron and nickel have proven useful in connection with the present invention and are readily commercially available. Several common metal catalyst alloys can include Fe—Ni, e.g., INVAR alloys, Fe—Co, Ni—Mn—Co, and the like. Currently preferred metal catalyst materials are Fe—Ni alloys, such as Fe-35Ni, Fe-31Ni-5Co, Fe-30Ni, and other INVAR alloys, with Fe-35Ni being most preferred and more readily available. One additional preferred metal catalyst includes cobalt and alloys thereof In addition, the catalyst materials for diamond synthesis can include additives which control the growth rate of diamond, i.e. via suppressing carbon diffusion, and also prevent excess nitrogen and/or oxygen from diffusing into the diamond. Suitable additives can include Mg, Ca, Si, Mo, Zr, Ti, V, Nb, Zn, Y, W, Cu, Al, Au, Ag, Pb, B, Ge, In, Sm, and compounds of these materials with C and B.

Similarly, catalyst materials suitable for cBN can include any catalyst capable of promoting growth of cBN from suitable boron nitride raw materials. Non-limiting examples of catalyst materials suitable for cBN synthesis can include alkali metals, alkaline earth metal, nitrides thereof, Al—Si alloys, and compounds thereof Several specific examples of such catalyst materials can include, without limitation, lithium, calcium, magnesium, and nitrides of alkali and alkaline earth metals such as Li₃N, Ca₃N₂, Mg₃N₂, CaBN₂, and Li₃BN₂.

The specific amount of sintering aid can vary; however, excessively high sintering aid content can result in formation of weak abrasive grits. Similarly, low concentrations of sintering aid can result in slow growth rates and/or poor crystal quality, i.e. high defect rates. As a general guideline, the sintering aid can comprise from about 10 vol % to about 50 vol % of the dewaxed abrasive precursor. During sintering, some of the sintering aid will tend to migrate out of the abrasive particles and is present in the final product in relatively small amounts. Further, the sintering aid content of the final product can be reduced by leaching with an acid or by other known techniques.

As mentioned above, the dough can include a binder. Typically, the binder can be any material which acts to adhere the abrasive particles together. Further, it is frequently desirable that the binder be a fugitive binder such that heating or other binder removal processes leave substantially no residue on the abrasive particles. Excessive or substantial amounts of residue or foreign material can interfere with sintering of the abrasive particles and reduce the quality of the final polycrystalline grits. Non-limiting examples of suitable binders can include polyethylene glycol, polyvinyl alcohol, wax, paraffin, naphthalene, polyvinyl butyral, phenolic resin, wax emulsions, acrylic resins, and mixtures thereof In one detailed aspect, the binder can be polyethylene glycol. Generally, the binder can comprise from about 10 vol % to about 50 vol % of the abrasive slurry. This range is only exemplary, as practical amounts of binder can depend on the method used to form the abrasive precursors and the specific materials used, as discussed further below.

Once the dough is created and formed into a sheet, the sheet can be placed on a stretchable surface. The stretchable surface can range from a thin sheet of stretchable material, to a thicker surface, such as a self-supporting surface. The surface must have the capacity to stretch in at least one direction, and preferably in two perpendicular directions. In one aspect, the stretchable surface can comprise or consist essentially of an organic material. The stretchable surface can, in one embodiment, include a thin sheet of an organic or a synthetic polymer. A non-limiting example of a stretchable surface is Mylar, or biaxially-oriented polyethylene terephthalate polyester film.

A particulate separating agent can optionally be distributed on the stretchable surface. A particulate separating agent can comprise a particulate material that serves the purpose of separating abrasive particles from nearby objects, including other abrasive particles. At the same time, the separating agent can comprise a material that aids in maintaining the integrity of an abrasive precursor assembly during sintering. A wide variety of materials can be suitable for this purpose such as, but not limited to, NaCl, Co, Cu, pyrophillite, dolomite, talc, metal oxides, hexagonal boron nitride (hBN), graphite, and combinations thereof. Currently preferred materials for use as a separating agent include NaCl, hBN, and graphite. The particulate separating agent can be of any particle size which allows for proper formation of polycrystalline abrasive particles. In one aspect, the particulate separating agent can have a particle size from about 1 μm to about 40 μm.

Optionally, the separating agent can further include a refractory metal, or refractory metal carbide which can be used to adjust the softness of the pressure medium. Non-limiting examples of suitable additives for the separating agent can include, but are not limited to, SiC, WC, TiC, and mixtures thereof The abrasive dough sheet can be divided into predetermined shapes to form a plurality of abrasive precursors. Such division can occur while the sheet is placed on the stretchable surface, or can occur while the sheet is placed on another surface and then transferred to the stretchable surface as a divided sheet. Once the sheet is divided into the abrasive precursors and placed on the stretchable surface, the stretchable surface can be stretched to separate the abrasive dough sheet and reveal separations among the abrasive precursors. This can be done in a manner so as to allow for even and predetermined separations among the abrasive precursors. With the separations revealed, particulate separating agent can be distributed in the separations. The particulate separating agent distributed among the abrasive precursors can be the same particulate separating agent as that placed on the stretchable surface. Optionally, excess separating agent can be can be removed. Once a desired amount of separating agent is distributed among the abrasive precursors and optionally along the top of the abrasive precursors, any excess separating agent can be removed by techniques that would be apparent to one skilled in the art, including scraping, directing streams of air or other gas at the excess separating agent, etc.

In an alternate embodiment, a rigid support, or otherwise support without a stretchable surface, can be used. In this case, the dividing of the sheet can produce a plurality of abrasive precursors separated by kerf lines, or cutting lines. Such separation may depend on the dividing process and apparatus used. It may be necessary to remove swarf produced in the cutting or dividing process to better reveal the kerf. Once the abrasive precursors are formed, they can be separated or coated with particulate separating agent. Particulate separating agent can be distributed in the kerf Such distribution can be better facilitated by small particle size of the particulate separating agent, and optionally by shaking or vibrating distribution of the separating agent. Particulate separating agent can optionally be distributed between the abrasive precursors and the rigid support. In such manner, an abrasive precursor can be coated with particulate separating agent, although it should be noted that any manner of effectively coating an abrasive precursor with the separating agent is within the scope of coating as described herein.

The step of dividing can, in one aspect, include cutting the abrasive dough sheet with a blade. Dividing the dough sheet can occur in a single step, such as with a bladed-stamp, or can include a plurality of steps. Alternatively, the abrasive dough sheet can be formed into abrasive precursors using a template. The template can be designed with slim blades that can divide the dough sheet. The template can be designed to form abrasive precursors of any shape, including square, rectangular, circular, triangular, hexagonal, cross-shaped, or any other practical shape. Typically, square or rectangular shaped abrasive precursors are desirable for production of abrasive polycrystalline grits. When blades are used to divide the dough sheet, the divisions can produce abrasive precursors having a surface with an inverse profile of a surface of the adjacent abrasive precursor.

Regardless of the method used to divide the abrasive dough sheet into abrasive precursors, the arrangement of abrasive precursors, specifically the separation between adjacent abrasive precursors, can involve a wide variety of spacings and abrasive precursor shapes. The spacing of the abrasive precursors can be dependant on the spacing provided by the stretching of the stretchable surface. In one embodiment, the abrasive precursors can be spaced having a pitch (center-to-center distance) which is about three times that of the abrasive precursor diameter. In another aspect, the edge-to-edge distance between abrasive precursors can be from about 0.5 to about 10 times the diameter of the abrasive precursors. The spacing between abrasive precursors is generally a compromise between yield of abrasive polycrystalline grits and quality of the abrasive polycrystalline grits. Specifically, as the abrasive precursors are placed closer together, there is a greater risk that adjacent grits will grow together. Those skilled in the art can choose materials and conditions which can minimize this affect based on the teachings disclosed herein.

Subsequent to forming the abrasive precursors and prior to sintering, the binder and other organic materials can be removed from the abrasive precursors in a dewaxing process. Dewaxing can be performed to increase the coherence and integrity of the abrasive precursors. Further, dewaxing can remove substantially all organic materials, e.g., binder, remaining stretchable surface, etc. Excessive residual organic materials can interfere with sintering and reduce the integrity and mechanical strength of polycrystalline grits produced thereby. Typically, dewaxing can be accomplished in a two step process including drying, or solvent removal, and then heating to dewax or remove the binder. This dewaxing process preferably provides sufficient heat to remove substantially all organic materials from the abrasive precursors. As a general rule, initial heating of the abrasive precursors to a temperature from about 90° C. to about 120° C., followed by heating to about 550° C. to about 700° C. can provide adequate results. Most often, dewaxing is performed under a vacuum with an inert atmosphere such as hydrogen, nitrogen, argon, or the like. Optionally, an additional oxidation step can be used to partially degrade the binder into smaller molecules, especially when polymeric binders are used. This additional step can be typically performed at an intermediate temperature around 350° C.

The separated abrasive precursors can be transferred to another surface. Such transfer can occur by any method known in the industry, including utilizing surfaces having adhesive. The abrasive precursors can be assembled, along with the stretchable surface, or without, into an abrasive precursor assembly. Transferring and transporting techniques can be used to assemble a multi-layered precursor assembly.

In an additional optional embodiment, a plurality of precursor assembly layers can be assembled to form a multi-layered precursor assembly. Typically, a single precursor assembly can contain dozens or hundreds of abrasive precursors, while a multi-layered precursor assembly can contain several tens of thousands of individual abrasive precursors. For example, typical reaction volumes for high pressure-high temperature (HPHT) devices can range from about 1 cm³ to about 1000 cm³. Further, diameters of the precursor assemblies can range from about 5 mm to about 10 cm, and typically from about 1 cm to about 10 cm, depending on the type of HPHT device used. Similarly, typical abrasive precursors can measure from about 0.5 μm to about 4 mm, and preferably from about 37 μm to about 1 mm. Additionally, the present invention also allows for production of ultra-fine polycrystalline grits having an average size from about 0.5 μm to about 2 μm. As such, the abrasive precursors typically occupy from about 1 vol % to about 50 vol % of the precursor assembly. For example, the number of abrasive precursors present on a single precursor assembly in some embodiments can range from about 10 to about 70,000, depending on the precursor assembly size, number of layers, and grit size.

In accordance with the present invention, the precursor assembly can be placed in a device capable of sintering the abrasive particles to form an assembly of sintered polycrystalline grits. Any device which is capable of producing sufficiently high pressures and temperatures to cause sintering of the abrasive particles can be used. Non-limiting examples of several suitable high pressure devices can include cubic presses, multi-anvil presses, belt presses, toroidal presses, piston-style presses, and the like. Sintering conditions can vary considerably from one material to another. For example, sintering pressures for diamond and cBN can range from about 4 GPa to about 7 GPa, and typically up to about 5.5 GPa in the case of diamond. Similarly, temperatures can vary depending on the material; however, for diamond superabrasive particles, temperatures of about 1300° C. are suitable, while for cBN, temperatures of about 1500° C. are typical. The sintered polycrystalline grits typically can correspond to the original abrasive precursors with slight changes in dimensions due to volume reductions during sintering. Thus, the assembly of sintered polycrystalline grits can include a plurality of discrete polycrystalline grits separated by consolidated separating agent.

Subsequent to sintering, the assembly of sintered polycrystalline grits can be removed from the sintering device. Individual polycrystalline grits can then be recovered using any number of techniques. Recovery of polycrystalline grits of the present invention can be readily accomplished by crushing, solvents, combinations thereof, or the like. When crushing the assembly, the pressure medium tends to fracture and break apart without damaging the polycrystalline grits. Similarly, solvents can be used to dissolve or otherwise weaken the pressure medium sufficiently to allow recovery of polycrystalline grits. Frequently, the pressure medium can be chosen so as to facilitate release of polycrystalline grits from the sintered assembly. For example, a separating agent of sodium chloride can be readily crushed and/or dissolved using heated aqueous solutions or other common solvents. In one aspect of the present invention, the recovered polycrystalline grits can have substantially unaltered surfaces such that the recovered polycrystalline grits can be used in abrasive applications without further modification of surfaces of the polycrystalline grits. Typically, the polycrystalline grits can be substantially unchanged in shape, surface roughness, and/or other properties by the recovery process.

Additional cleaning steps can sometimes be required to remove residual debris, metals, or pressure medium from the polycrystalline grits. However, such cleaning steps typically do not change the shape, surface properties, or integrity of the polycrystalline grits. Similarly, the polycrystalline grits can be burnish milled or otherwise treated to remove undesirable defects or protrusions, e.g., residual metals, debris, or sintered bodies resulting from defects in the formation process. Alternatively, residual sintering aid inclusions can be leached out by soaking in acid over an extended period of time. Removal of sintering aid from the final abrasive polycrystalline grits helps to improve thermal stability. In many cases, the polycrystalline grits of the present invention can be thermally stable up to temperature of about 1100° C.

A wide range of polycrystalline grit sizes and shapes can be produced, as discussed in connection with formation of the abrasive precursors. Typically, the polycrystalline grits can have an average size from about 400 mesh to about 10 mesh, and preferably from about 325 mesh to about 18 mesh, depending on the specific intended application. The methods of the present invention further allow production of large amounts of polycrystalline grits having highly uniform size distributions. In accordance with the present invention, the polycrystalline grits can have substantially uniform sizes and shapes to within several micrometers. However, as a broad matter, the polycrystalline grits can have a uniform size distribution characterized by a size distribution of less than about 50 μm, preferably less than about 10 μm, and in many cases less than about 5 μm. Preferably, the final polycrystalline grits can comprise greater than 80 vol % abrasive, e.g., diamond, cBN, or the like.

In accordance with the present invention, the polycrystalline grits can have substantially uniform shapes and sizes. As a result, polycrystalline grits can be supplied having substantially uniform sizes without the time and cost of segregating different size grits or otherwise sorting the grits. Additionally, substantially uniform polycrystalline grits can improve performance in abrasive applications by allowing for microfracturing of the grits rather than macrofracturing. Specifically, the polycrystalline structure allows small portions of the grit to fracture without causing catastrophic failure of the entire grit. Additionally, the constant micro fracturing of each grit allows for a continual renewal of sharp edges on the grit which helps to maintain cutting speed. Further, polycrystalline grits of the present invention tend to be relatively rough as compared to single crystal diamonds. This increased roughness allows for improved bonding with various tools.

In an additional aspect of the present invention, a substrate can be included in the abrasive precursor assembly. The substrate can include any suitable material which can be useful to retain the abrasive precursors in the desired arrangement during formation of the abrasive precursors and sintering of the abrasive particles. The substrate can be formed of any material which has sufficient integrity to allow formation of abrasive precursors thereon. Thus, almost any material can be suitable including, but not limited to, metal foils, metal plates, films, polymeric sheets, paper, or the like. In some embodiments, the substrate can comprise a sintering aid which provides an additional source of sintering aid to the abrasive precursors. Suitable substrates can comprise a metal or non-metal material, generally provided in the form of a thin disk or sheet. Non-limiting examples of suitable substrate materials can include cobalt, nickel, iron, copper, sodium chloride, hexagonal boron nitride, graphite, stainless steel, corundum, and alloys, mixtures, or composites thereof Additional materials which can be suitable for use in the substrate can include titanium, tungsten, tantalum, nickel, zirconium, zinc, vanadium, chromium, steel, silicon carbide, quartz, silicon nitride, boron nitride, tungsten carbide, titanium carbide, and zirconium carbide, zinc oxide, zirconia, aluminum nitride, titanium nitride, and zirconium nitride, and mixtures, alloys or composites thereof. Typically, the substrate can have a thickness from about 30 μm to about 500 μm, although thicknesses outside this range can also be used. For convenience in processing, the substrate can be provided as a single sheet which is then cut or otherwise separated into smaller segments subsequent to formation of the abrasive precursors thereon. The smaller segments can be sized for placement in a particular high pressure device. Optionally, the substrate can be cut into smaller segments prior to formation of abrasive precursors thereon. Once the abrasive particles are sintered, they can be removed from the substrate.

A method for forming polycrystalline grits can therefore include forming a plurality of abrasive precursors including binder and a plurality of abrasive particles, coating the plurality of abrasive precursors with particulate separating agent, and sintering the abrasive precursors to form polycrystalline grits.

Forming polycrystalline grits according to the methods outlined herein allows for the creation of polycrystalline grits that are more impact resistant than single crystal grits. The polycrystalline grits can exhibit higher thermal stability than their single crystal counterparts. Furthermore, polycrystalline grits are capable of micro-chipping which prevents dullness as with a single crystal. Therefore, polycrystalline grits can have wide-spread application in fields such as sawing, drilling, and slicing.

Additionally, the disclosed methods allow for full use of the limited volume for sintering, such as with high pressure high temperature processing. Further, the abrasive particle material is conserved by the use outlined herein, and man-power to arrange and effectively separate abrasive precursors is greatly reduced. Such efficiency in the noted areas can greatly reduce the per-grit-cost, while providing superior polycrystalline grits having predetermined shapes. Specifically, the disclosed method provides a way to form a plurality of abrasive precursors in desired shapes such that can be uniformly separated and divided. Using the particulate separating agent allows for shorter separating distance, and therefore allows for a greater number of abrasive precursors to fit into the reaction chamber. Additionally, the methods allow for formation of a plurality of abrasive precursors, as opposed to individual shaping and placing of abrasive precursors. Such efficiency not only permits viable and commercial production of a plurality of abrasive precursors, but allows for viable and commercial production of uniform and smaller abrasive precursors.

EXAMPLE 1

Diamond powder of 2-4 μm is etch cleaned with molten fused salt such as NaOH, or eutectic mixture of NaCl—KCl—CaC₁₂. The diamond powder is then thoroughly rinsed with DI water. The cleaned diamond powder is used in a mixture, along with 5 wt % silicon powder having an average particle size of 10 μm and 10 wt % wax binder. The mixture is thoroughly mixed to produce a dough. The dough is compacted repeatedly between two steel rollers to form a pull sheet of about 1 mm thick. The pull sheet is transferred to a tray having a sheet of Mylar that is sprinkled with fine graphite powder having an average particle size of about 20 μm. The graphite is natural graphite and has a high degree of graphitization and is soft to the touch. A set of parallel roller knives are used in perpendicular directions to slit the pull sheet into a plurality of cubic abrasive precursors. The Mylar is pulled in the directions perpendicular to the slits to reveal separations among the cubic abrasive precursors. Graphite powder is sprinkled in the separations as well as on top of the abrasive precursors. Extra graphite powder is blown off with dry nitrogen gas. At this point, the abrasive precursor cubes are about 1 mm in size and are each surrounded by a graphite powder coating.

A circular core cup is used to cut the pull sheet cubes further to form 40 mm circles. These circles are stacked up to about 35 mm tall. A graphite mold is used to consolidate the stack to form a compact cylinder by hot pressing at 400° C. and 40 MPa. As a result, most of the binder is removed from the precursors, and the precursor shrinks about 10 V %. The consolidated cylinder is placed at the center of a high pressure assembly that is fitted with six anvils of the cubic press. About 2000 tons of pressure is applied to the high pressure assembly to reach a pressure of about 5.5 GPa. Subsequently, electrical current is passed around the core to heat the reaction charge to about 1400° C. At that point, silicon powder melts and acts to help sinter the abrasive precursors. After about 20 minutes of sintering, the cubes shrink to about 0.5 mm (30/40 mesh). Each sintered cube is separated by graphite powder that is relatively inert to the reaction except for a possible minimal amount of attachment on sintered cube surfaces due to the reaction with silicon.

After cooling and decompression, the charge is retrieved from the high pressure assembly and it is gently ball milled to separate the sintered cubes. Subsequently, the freed cubes are boiled in concentrated sulfuric acid to remove any attached graphite on the surface. The result is full sintered polycrystalline diamond cubes with SiC/Si inside. Unlike single crystal diamond grits that may be degraded above 700° C. due to the presence of internal catalyst material, the polycrystalline diamond cubes are thermally stable up to even 1200° C.

EXAMPLE 2

Same as Example 1, except cobalt powder is used in place of silicon powder. The sintered polycrystalline grits are not as thermally stable, but the overall diamond volume percent can be higher due to the higher sintering efficiency of molten cobalt liquid.

EXAMPLE 3

Same as Example 1, except a core of about 90 mm in diameter and 40 mm in thickness is fabricated and the ultrahigh pressure used for sintering is a belt apparatus instead of a cubic press.

EXAMPLE 4

Same as Example 1, except the sheet is divided on a substrate and then transferred to a Mylar film, using adhesive coatings (stronger coating on the graphite-covered Mylar than on the transportation sheet). Once the sheet is transferred to the Mylar film, it is stretched to reveal separations among the abrasive particles.

EXAMPLE 5

Micron sized cubic boron nitride powder is thoroughly mixed with micron sized TiN (5 vol %) and AlN (5 vol %) in an attritor mill. To the mixture is then added an additional 20 vol % of acrylic binder that is diluted with ethanol. The binder is fully mixed in with the use of the attritor. The dough-like blend is then compacted between two stainless steel rollers a plurality of times, with a PET sheet held underneath as the support until the dough forms a sheet of about 0.5 mm thick. The sheet is then cut by a set of rotating dicing wheels to form parallel grooves of about 50 microns apart. The cut precursor is turned 90 degrees and another set of grooves are cut. The result are abrasive precursors with the edge length of about 0.5 mm. Graphite powder is then sprinkled from a shaking sieve into the kerf and onto the surface of the abrasive precursors.

A stainless steel tube with a sharp rim is used to form disks of about 40 mm in diameter of the layer of separated abrasive precursors with the PET sheet. The disk with the PET is placed on the remaining sheet that is covered with graphite powder. The PET is withdrawn and the tube is then pressed down to cut the second layer of abrasive precursors. In this case, the graphite powder separates the two layers. This process is repeated a plurality of times until the stack height reaches about 50 mm. The stack inside the tube is then baked at 300° C. for 2 hours to dewax the abrasive precursors and remove the binder. Subsequently, the stack inside the tube is compressed in a hydraulic press to reach a height of 30 mm. The compressed stack is punched out the tube that can be reused for making the next stack of abrasive precursor layers.

The dewaxed and consolidated stack of abrasive precursors is then pressed by a cubic press to reach about 6 GPa and is heated to about 1400° C. for about 20 minutes. The micron cBN is fully sintered with the aid of TN and AlN. The recovered mass is mildly milled in a rotating ball mill to break up the sintered abrasive particles. The separated abrasive particles are then soaked in warm sulfuric acid until the sintered polycrystalline cBN is clean.

The polycrystalline abrasive precursors so formed are then optionally coated with Ti by CVD and then brazed onto a turbo grinder. The grinder can be used to trim welding bulges in a ship yard. Normally the steel cannot be ground by diamond grits, however, the single crystal grits of cBN are too small to be effective. The sintered polycrystalline cBN as formed herein can work well for this and other similar applications. Furthermore, polycrystalline cBN abrasive particles can also be formed into wire saws to slice steel beams for construction works.

Of course, it is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements. Thus, while the present invention has been described above with particularity and detail in connection with what is presently deemed to be the most practical and preferred embodiments of the invention, it will be apparent to those of ordinary skill in the art that numerous modifications, including, but not limited to, variations in size, materials, shape, form, function and manner of operation, assembly and use may be made without departing from the principles and concepts set forth herein. 

1. A method of forming polycrystalline grits, comprising: forming an abrasive dough into a sheet, said abrasive dough including a plurality of abrasive particles; placing at least a portion of the sheet on a stretchable surface including particulate separating agent; dividing the sheet into predetermined shapes to form a plurality of abrasive precursors; stretching the stretchable surface sufficient to reveal separations among the plurality of abrasive precursors; distributing particulate separating agent in the separations; and sintering the abrasive precursors to form a plurality of polycrystalline grits.
 2. The method of claim 1, wherein the abrasive particles are selected from the group consisting of diamond, nanodiamond, cubic boron nitride, silicon carbide, quartz, corundum, silicon nitride, boron nitride, tungsten carbide, titanium carbide, zirconium carbide, zinc oxide, zirconia, alumina, aluminum nitride, titanium nitride, zirconium nitride, and mixtures or composites thereof.
 3. The method of claim 1, wherein the abrasive particles are superabrasive particles.
 4. The method of claim 3, wherein the abrasive particles are diamond particles.
 5. The method of claim 1, wherein the abrasive particles include a mixture of superabrasive and non-superabrasive particles.
 6. The method of claim 1, wherein the abrasive dough further comprises a sintering aid.
 7. The method of claim 1, further comprising a step of dewaxing prior to the step of sintering.
 8. The method of claim 1, wherein the particulate separating agent comprises a member selected from the group consisting of NaCl, Co, Cu, hexagonal boron nitride, graphite, and combinations thereof.
 9. The method of claim 8, wherein the particulate separating agent includes graphite.
 10. The method of claim 1, wherein the particulate separating agent has a particle size from about 1 μm to about 40 μm.
 11. The method of claim 1, wherein the step of dividing the sheet into predetermined shapes occurs while the sheet is placed on the stretchable surface.
 12. The method of claim 1, wherein the step of distributing particulate separating agent in the separations includes removing excess separating agent.
 13. The method of claim 12, wherein removing excess separating agent includes directing a stream of air or other gas at the excess separating agent.
 14. The method of claim 1, wherein the stretchable surface includes an organic material.
 15. The method of claim 1, wherein the step of dividing the sheet into predetermined shapes to form a plurality of abrasive precursors creates a plurality of abrasive precursors, each having a surface with an inverse profile of a surface of another abrasive precursor.
 16. A method of forming polycrystalline grits, comprising the steps of: forming a plurality of abrasive precursors, including a binder and a plurality of abrasive particles; coating the plurality of abrasive precursors with a particulate separating agent; and sintering the abrasive precursors to form polycrystalline grits.
 17. The method of claim 16, wherein the abrasive particles are diamond particles.
 18. The method of claim 16, wherein the particulate separating agent has a particle size from about 1 μm to about 40 μm.
 19. An abrasive precursor assembly, comprising: a plurality of abrasive precursors including a sintering aid, abrasive particles, and binder, arranged on a stretchable surface, said stretchable surface stretched sufficient to reveal separations between the abrasive precursors; and a particulate separating agent in the separations and between the abrasive precursors and stretchable surface.
 20. The abrasive precursor assembly of claim 19, wherein the abrasive particles are diamond particles. 